1. Field of the Invention
The present invention relates to a drug activation system efficient for targeted drug delivery into the human body.
2. Description of the Related Art
Many anti-cancer drugs kill dividing cells to prevent uncontrolled growth of malignant cells. Systemic application of such drugs can also damage healthy cells resulting in serious side effects and the development of therapy-induced tumours. Directed enzyme prodrug therapy (DEPT) is based on the targeted delivery of enzymes directly to the malignant growth, where the enzymes convert a harmless prodrug into a cytotoxic drug.
To try and further increase tumour selectivity, research has recently focused on using tumour specific antigens to target the drug/prodrug directly to the solid tumour as disclosed for example in Kratz et al. (Kratz, F., Muller, I. A., Ryppa, C. & Warnecke, A., Chem Med Chem 3, 20-53, 2008).
Another selective approach targets the prodrug-activating enzyme or its encoding gene to the tumour before administering the prodrug, using carriers such as antibodies, viruses, cationic lipids, peptides, or naked DNA, as disclosed for example in Bagshawe (Bagshawe, K. D. in Prodrugs, Vol. V., eds. V. J. Stella et al., Springer New York, N.Y.; 2007) or in Schepelmann and Springer. (Schepelmann, S. & Springer, C. J. in Current Gene Therapy 6, 647-670, 2006), or in Behr (Behr, J.-P., Bioconjugate Chem 5, 382-389, 1994), or in Wadhwa et al. (Wadhwa, M. S., Collard, W. T., Adami, R. C., McKenzie, D. L. & Rice, K. G., in Bioconjugate Chem 8, 81-88, 1997).
The limitations of the above mentioned methods are the scarcity of tumour specific antigens, the immunogenicity of the drug-carrier combination, and the inefficient expression of enzymes from the targeted DNA.
A different approach for targeted chemotherapy is the use of magnetic nanoparticles (MNPs), which can be directed to the tumour using an external magnetic field. Additionally, treatment localisation and progress can be monitored using real-time Magnetic resonance (MR) imaging. The most common MNPs investigated for cancer therapy has been iron oxide nanoparticles, due to their low toxicity, and approval by the Food and Drug Administration. Such methods have been described for example in Alexiou et al. (Alexiou, C. et al. in Eur Biophys J Biophy 35, 446-450, 2006), or in Kohler et al. (Kohler, N., Sun, C., Wang, J. & Zhang, M. in Langmuir 21, 8858-8864, 2005), or in Kohler et al. (Kohler, N. et al. in Small 2, 785-792, 2006), or in Weissleder et al. (Weissleder, R. et al. In American Journal of Roentenology 152, 167-173, 1989) or in Ros et al. (Ros, P. R. et al. in Radiology 196, 481-488, 1995), or in Reimer and Tombach (Reimer, P. & Tombach, B. in Eur J Radiol 8, 1198-1204, 1998).
Research into the use of bacterial nitroreductases (NTRs) in DEPT has focused on the use of enzymes such as Escherichia Coli (E. coli) to activate dinitrobenzamide prodrugs at solid tumours, as described for example in Searle et al. (Searle P. F., M. J. Chen, L. Hu, P. R. Race, A. L. Lovering, J. I. Grove, C. Guise, M. Jaberipour, N. D. James, V. Mautner, L. S. Young, D. J. Kerr, A. Mountain, S. A. White, and E. I. Hyde, in Clinical and Experimental Pharmacology and Physiology, 31, 811-816, (2004). However, viral vectors still lack specificity and the slow turnover rate of the studied prodrugs limits the therapeutic efficacy of this NTR/prodrug combination in DEPT as explained for example in Astrimayer et al. (Astridmayer, R. Francis, S. K. Sharma, Berendtolner, C. Springer, J. Martin, G. Boxer, J. Bell, A. Green, J. A. Hartley, C. Cruickshank, Juliewren, K. Chester, and R. H. J. Begent. in Clinical Cancer Research. 12: 6509-6516, (2006).
Attempts to improve the kinetic abilities of these enzymes by site-directed mutagenesis were reported by Race et al. (Race P. R., A. L. Lovering, S. A. White, J. I. Grove, P. F. Searle, C. W. Wrighton, and E. I. Hyde, in Journal of Molecular Biology. 368, 481-492, (2007).
The synthesis of novel prodrugs has also been reported in Anlezark et al. (Anlezark G. M., R. G. Melton, R. F. Sherwood, B. Coles, F. Friedlos, and R. J. Knox. In Biochemical Pharmacology. 44: 2289-2295 (1992) or in Asche et al. (Asche C., P. Dumy, D. L. Carrez, A. Croisyb, and M. Demeunyncka. in Bioorganic and Medicinal Chemistry Letters. 16: 1990-1994, (2006). This has resulted in substantial improvements, but the enhanced kinetic parameters may still limit clinical efficacy. These studies suggest that other bacterial species may have increased catalytic efficiencies better suited for DEPT.
There is thus a need to improve the delivery of drugs directly to the diseased cells.